Data from the U.S. Geological Survey suggests that this morning’s Chile quake tied in fifth place with an 8.8 quake that hit Ecaduor and Bolivia in 1906. Only four quakes have been bigger since 1900. The largest was a 9.5 magnitude event that struck Chile in 1960, causing 1,655 fatalities, leaving 2 million homeless, and triggering a tsunami that killed people in Hawaii, Japan and the Philippines.

Scores of countries around the Pacific Ocean are bracing for a tsunami unleashed by the latest quake, and which is now speeding across the ocean at 550 miles per hour, or the speed of a jet plane.

“A tsunami has been generated that could cause damage along coastlines of all islands in the state of Hawaii,” noted the U.S. government’s tsunami warning center in Hawaii.

Tsunami-causing quakes usually occur where shards of the earth’s crust – tectonic plates – meet. Magma rises from deep inside the earth, causing the plates to move. They slip-slide past each other, sometimes get stuck, then jerk forward again, producing a quake.

According to the USGS, the Chile earthquake occurred at the boundary between the Nazca and South American tectonic plates. The two plates are converging at a rate of 80 mm per year, with the Nazca plate moving down and landward below the South American plate.

The huge disruption in the sea floor acted like a giant wave machine, displacing a large amount of water and triggering tsunami waves. The resulting undulations aren’t usually detectable by ships since the crests often measure less than three feet in height and are hundreds of miles apart. But the force of a tsunami becomes apparent in shallower water.

As it approaches the coast, a tsunami slows down to about 20 to 30 miles an hour. It is now at its most dangerous: All its energy gets compressed into much less depth, and the height of the wave can dramatically increase.

When a tsunami wave hits a coastline, its trough can temporarily expose the sea floor, though water quickly floods the area again. Such an event can trigger powerful and unpredictable currents along the shore, and debris picked up by the wave can boost its destructive power.

Some 95% of the world’s earthquakes occur in the Pacific Ocean; that’s why the devastating earthquake-triggered tsunami that occurred in the Indian Ocean on Boxing Day 2004 took so many people by surprise. The Pacific Rim has long been ringed with early-warning systems intended to detect an imminent tsunami in time to allow people to flee to higher ground.

Coastal Chile has a history of massive earthquakes. Since 1973, there have been 13 events of magnitude 7.0 or greater, according to the USGS. The February 27 event originated about 230 km north of the source region of the magnitude 9.5 earthquake of May 1960 – the biggest temblor anywhere in at least 200 years. That quake spawned a tsunami that engulfed the Pacific Ocean, the USGS says.

Nor is the danger over for residents of Chile who live near the quake’s epicenter. “A large vigorous aftershock sequence can be expected from this earthquake,” warns the USGS.